Preheat the oven to 375 °F (190 °C) and line 1-2 baking trays with parchment paper.

Heat a sauté pan over medium heat and melt the butter. Add the onion and sauté until the onion is translucent, about 5 minutes. Stir in the garlic and lemon zest. Squeeze out the thawed spinach well and add to the pan, stirring well to break it down, and stir to warm it through. Spoon the spinach into a bowl, stir in the feta and season to taste.

Lay out a sheet of filo and brush evenly with the melted butter. Place a second sheet of filo on top and brush it with butter. With a knife or pastry wheel cut the pastry into 6 strips, starting from a long side. Spoon a little of the filling at the base of each strip. Fold the filo strips into triangles starting at the end with the filling, sealing in the filling as you fold up each triangle. Repeat with the remaining filo sheets and filling. Brush the pastries with eggwash and bake for 15 to 18 minutes until golden brown. Serve the pastries warm. Alternatively, the pastries can be assembled and chilled until ready to bake.